The Duration of the Cretaceous

The Cretaceous Period was one of high sea-levels and warm temperatures. It ended 65 million years ago; but began 142 or 144 million years ago; depending on the classification chosen.

The following discussion covers:

• The Beginning of the Cretaceous
• The End of the Cretaceous

Boundaries of the Cretaceous

Many people now know that the Cretaceous, and the reign of the dinosaurs, ended with a 'Big Bang' - when a meteorite slammed into the Earth at Chicxulub (on the Yucatan Peninsula, Mexico) 65 million years ago. The impact caused a 'mass extinction' of about 85% of the species (and xx% of the genera) on the Earth at the time; the dinosaurs were the biggest to go, but organisms down to the [micro-level] were also [wiped out].
Less commonly known is that many of the dinosaurs were already in decline - the meteorite impact was the "final straw" to a downward trend in their survival - albeit a hefty 'straw'.

The Beginning of the Cretaceous

The beginning of the Cretaceous is less well-known; and indeed, less well-defined. Different schools use a different base for the Period, so it cannot yet be stated whether the Cretaceous began 144 Ma ago (using the Mediterranean boundary between the Tithonian and Berriasian stages), or 142 Ma ago.
(The latter uses high-latitude boreal zonations in offshore Norway and equates to the boundary between Volgian and Ryazanian in Siberia This is an ammonite zone, and may coincide with the top of the British Portlandian, so is generally more useful for local zonation; but the world-standard appears to be veering toward the Mediterranean definition; although a confirmed scientific [definition] has not yet been reached (2002).

The two standards are based on the fact of 'provincialism' in the fossils used to date stage boundaries. At the end of the Jurassic there were two broad zones, the 'Boreal' and the 'Tethyan' which had different suites of fossils, though sometimes, e.g. at Speeton in Yorkshire alternations of Boreal and Tethyan fauna are found.
As there was not a global event (such as terminated the Cretaceous) the changeovers in fauna that are used to characterise a stage boundary occurred at different times in the different provinces (and involved different fauna suites).

Geologists in the different areas all like to claim that their event is the one to use; each certainly use that that is suitable to their local conditions within their own studies and papers.

The End of the Cretaceous

However; the extinction pattern had a number of variations, and some as-yet unsolved mysteries. Amongst the oceanic starting with nanoplankton, the (calcareous) coccoliths and foraminiferas suffered greatly, but (siliceous) diatoms and dinoflagellates were not much affected. Through the marine food chain up to the reptiles there was devastation - [but sharks did not suffer much ?]
On land, the extinctions were also variable. Dinosaurs were totally eradicated, but turtles, crocodiles, salamanders, and small mammals (excepting marsupials) survived almost unaffected.

The end of the Cretaceous (and also, by definition, the end of the secondary era, the Mesozoic) is now sharply defined, by both 'relative' timescales (the the extinction of fossil fauna), and 'absolute dating' of an iridium layer that appears worldwide, and was a result of the meteoritic impact.
Until very recently, it was believed that this layer is nowhere preserved in Britain, as all the rock of that age had apparently been eroded. However; there is now a suggestion that it may be preserved in small amounts, under the basalts of the Inner Hebridian Islands.
The Chalk that would have preserved the signs in the South have certainly been eroded. The nearest Chalk in which the boundary layer can be seen in Maastricht in the Netherlands.

The Cretaceous period thus lasted either 79 or 77 million years.

For our purposes, it is immaterial; the Cretaceous followed on the from the Jurassic, dinosaurs continued to wonder the land, and its end is marked in the stratigraphic record in many places around the world by a distinct 1cm-thick layer of clay, rich in iridium - a rare element on Earth, but common in meteorites.

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For any comments, suggestions or contributions, please e-mail me at: portsdown@bbm.me.uk